ResolutionPlease check the following things:
1) 127.0.0.0/8 network is added to white list in Plesk CP (Server-> Mail -> White List)
2) Make sure that /usr/sbin/sendmail and /usr/lib/sendmail are a symbolic links to Plesk sendmail binary (/var/qmail/bin/sendmail or /etc/alternatives/mta)
3) If doesn't help - please check out if whole mail system works 1387. For example try to send mail via webmail or "mail" shell command. Trace down delivering stages through the $PRODUCT_ROOT_D/var/log/maillog.
4)Rebuild qmail configuration with help of $PRODUCT_ROOT_D/admin/sbin/mchk utility 944.
Please remember to restart qmail after running mchk.
5) If doesn't work anyway check psa-qmail rpm for integrity:
~# rpm -V psa-qmail
If psa-qmail package's files are modified, reinstall 'psa-qmail' and 'drweb-qmail' (if DrWeb is installed on the server) RPM packages. The rpm packages may be downloaded from website http://autoinstall.plesk.com/. Locate your Parallels Plesk Panel version, operating system and architecture. The files are locates in subdirectories base/ and opt/drweb/. For example for Parallels Plesk Panel 8.6, CentOS5, 32bit:
Select your own version.